Method of heat-treating railroad wheels



Oct. 27, 1964 F. J- DEWEZ, JR

METHOD OF HEAT-TREATING RAILROAD WHEELS Filed April 20, 1962 INVENTORFERNA/VD J. DEWEZ, Jn r fizz n Attorney United States Patent 3,154,441METHOD OF HEAT-TREATING RAILROAD WHEELS Fernand J. Dewez, Jr., Irwin,Pa., assignor to United States Steel Corporation, a corporation of NewJersey Filed Apr. 20, 1962, Ser. No. 189,077 4 Claims. (Cl. 148-143)This invention relates to a method for heat-treating metal Wheels and,in particular, to a heat-treatment for steel railroad wheels thatprovides improved resistance to plate-fatigue and explosive failures.

In service, trains are usually stopped by applying brake shoes directlyagainst the tread of the wheel. The frictional heat resulting from thisoperation produces temperature gradients within the rims of the wheelsthat cause the formation of circumferential tensile stresses in therims. Thermal cracks form initially as a result of these stresses. Ifsubsequent tensile stresses are sufficiently high and if thermal cracksare present, a sudden propagation of the thermal cracks occurs, commonlyknown as explosive failure. The explosive failure may propagate in aradial direction through the rim, plate, and hub or circumferentially inthe plate and join other thermal cracks in the rim. Explosive failuresoften result in train derailments.

Plate fatigue failures usually develop at the back-rim- 7 plate filletand at the front-hub-plate fillet (known as the critical plate fillets).These failures are caused primarily by repeated tensile stresses thatare produced at these critical plate fillets by thermal expansion of therim during braking. Cyclic stresses produced in the plate by the wheelrolling on the rail contribute to these failures, but these stresses arenot sufficient to cause failure in the absence of tensile stressesproduced by braking. The plate fatigue failures may propagatecircumferentially in the plate or join thermal cracks in the rim. Platefatigue failures can also result in train derailments.

Industry has in general employed two methods of heattreating railroadwheels to increase hardness and partially control residual-stresspatterns, as a means for obtaining improved resistance to explosive andplate fatigue failures. Both heat-treatments involve first'austenitizingthe wheels at a suitable temperature for approximately one hour. andthen quenching. In one method the quenching step comprises arim-quenching treatment, commonly known as rim toughening, and in theother method the wheel is rotating the wheels while in a verticalposition with only .he lowest portion of the wheel immersed in water toI the rim-plate edge. In oil quenching, the entire wheelis quenched byimmersion in an oil bath. Wheels which have been quenched in eithermanner from the austenitiz 'ing temperature are then tempered at asuitable subcritical temperature for approximately two hours andslow-cooled in pits for about twenty-four hours. In the rim tougheningmethod only the rims of the wheels are hardened; whereas, the rims,plates, and hubs of oilquenched wheels are hardenedl Both methods ofheat-treatment produce over-all hoop compressive stresses in the rim ofthe wheel, but these "compressive stresses are considerably higher inrim-toughened Wheels than in oil-quenched wheels. Residual hoopcompressive stresses in the rims of railroad wheels are desirablebecause they offset the thermally developed tensile stresses thatproduce thermal cracking and explosive failure. Residual radial tensilestresses are present at the critical plate fillets of rim-toughenedwheels, but residual radial compressive stresses are present at these3,154,441 Patented Oct. 27, 1964 locations in oil-quenched wheels.Residual radial compressive stresses at the critical plate fillets aredesirable, because they offset the tensile stresses that cause platefatigue failure. However, residual radial tensile stresses areundesirable, because they add to the stresses that cause plate fatiguefailure.

Results of service tests and full-scale laboratory tests havedemonstrated that rim-toughened Wheels have better resistance toexplosive failure than oil-quenched wheels. However, the service testshave demonstrated that the oil-quenched wheels have better resistance toplate fatigue failures than do rim-toughened wheels. Because platefatigue failures occur less often than explosive failures, the use ofrim-toughened wheels has generally been more satisfactory than the useof oil-quenched wheels. However, wheels are needed with betterresistance, both to explosive failure and to plate fatigue failure, thanthe conventional rim-toughened wheels.

It is therefore a principal object of this invention to provide a methodof heat-treating steel wheels to impart improved residual-stresspatterns.

A related object of this invention is the provision of a method oftreating steel wheels which permits control of the residual-stresspatterns developed.

A further more specific object of this invention is the provision of amethod of differentially quenching steel wheels from sub-criticaltempering temperatures to produce desired stress patterns.

Yet another object of this invention is to provide a method of improvingthe stress patterns in the plate porplate and hub of a steel wheel fromthe tempering temperature, preferably with water spray, after the wheelhas been previously quenched in a conventional manner from theaustenitizing temperature, desirable controlled residual-stress patternsare produced. Further, because the quenching is from below the criticaltemperature, the quenching rate can be varied without affecting themicrostructure or other physical properties of the steel wheel developedat the tempering temperature.

Referring now to FIGURES 1 and 2, an apparatus suitable for quenchingwheels according to this invention is shown supporting a wheel W inposition for quenching.

The wheel W includes a rim portion R, a plate P and a hub H. The rimportion R conventionally has a flange F, and for convenience, the plateP and hub H will hereafter be referred to collectively as the plate-hubportion. The wheel W is supported in upright position by a pair ofrollers 10 each having a groove 11 adapted to engage the flange F of thewheel. One of the rollers 10 is driven by a motor 12 and the otherroller is freely rotatable. The rollers 10 are journalled by bearings 13in a frame 14 which defines a water catching trough 16.

A first U-shaped spray head pipe 20 is provided which partiallysurrounds the rim of the wheel and has a plurality of openings 21arranged to spray liquid onto the rim portion R of the wheel. A pair oflateral spray heads 22 are arranged on opposite sides of the wheel andare connected to a central header 24. Each of the spray heads 22 hasaplurality of nozzles 25 adapted to direct liquid spray onto the plateand hub of the wheel.

The wheel to be heat-treated is heated to the tempering temperaturewhich must be at least 600 F. in order to provide the necessary physicalproperties but must be less than the lower critical temperature of thesteel, the preferable temperature being between 850 F. and 950 F. Whenthe wheel has been heated to this tempering temperature it is placed onthe rollers and the motor 12 is started and regulated to rotate thewheel W at about 60 r.p.m. The rim R and the plate-hub portion are thenspray quenched as the wheel is rotated. Two wheels which had previouslybeen rim-toughtened by rimquenching from above the austenitizingtemperature were differentially quenched from between 850 F. and 950 F.according to this invention. The quenching of the first wheel wasinitiated on the rim portion with water being sprayed from the U-shapedpipe 20 onto the rim at a rate of about 10 gallons per minute as thewheel was rotated. Five seconds after the quenching of the rim hadstarted water was sprayed from the nozzles 25 onto the plate-hubportion. Water was also fed from these nozzles at a total rate of 10gallons per minute. The quenching of the rim from the U-shaped nozzle 20was discontinued two minutes after it started with the rim thereafterair cooling; and the plate-hub portion was quenched for five minutes. Inthis way the plate-hub portion was quenched to ambient temperaturebefore the rim portion reached ambient temperature. The stressesdeveloped by this quench are shown in Table I below. The other wheel wassimilarly treated except that the quenching of the plate-hub portion wasdelayed for fifteen seconds instead of five seconds, the other times andrates being the same. This also resulted in the plate-hub portionreaching ambient temperature before the rim portion. The results of thistest are also shown in Table I below. For comparison, Table I also showsstresses developed in a conventionally rim-toughtened wheel and aconventionally oil-quenched wheel. The stresses in the rim are measuredby the radial saw-cut-strain test. In this test 2 inch gauge lengths aremarked tangentially on both rim faces. The rim is then machined as aconcentric section from the wheel and a radial section is sawed frombetween the gauge lengths. The change in gauge length is a measure ofthe overall hoop strain in the rim. Compression or rim closure stress isindicated by a minus sign and tension or rim opening stress is indicatedby a plus sign. The radial stresses at the back rim-plate fillet F1 andfront hub-plate fillet F2 are determined by cementing 45 rectangularrosette electrical-resistance strain gauges at these locations. Theprincipal radial residual stresses are calculated from stress relaxationmeasurements that are made after sections containing the gauges aremachined from the wheel.

TABLE I Wheel Treatment The most desired stress patterns in the rim,i.e. stress patterns which are least susceptible to explosive failure,are high compressive stresses in the rim. The more negative the valuefor the rim stresses the more desirable as far as explosive failure isconcerned. Also, the higher the compressive stresses at the backrim-plate fillet and the front hub-plate fillet the more resistant thewheel will be to plate fatigue failure. These are also designated byminus figures and hence, the more negative this value the more desirableit is. It can be seen from Table I above that the differential quenchingwith a fifteen seconds delay yields greater compressive hoop stresses inthe rim than are obtained with rim-roughened wheels subjected toconventional slow-cooling from tempering temperature. Also, substantialimprovements in the stress values at the back rim-plate fillet and fronthub-plate fillet are obtained. With the five seconds delay the hoopcompressive stresses are not quite as favorable as those developed byconventionally rim-toughening and slowcooling but an extraordinaryincrease is obtained in the desirable stresses at the two criticalfillets. Also, the wheel quenched with a five seconds delay has betterhoop compressive stresses in the rim than the conventionallyoil-quenched wheel slow-cooled from the tempering temperature and aboutthe same or slightly better stresses at the critical fillets. Thus, itcan be seen that by differentially quenching the rim and the plate-hubportion of the wheel from the tempering temperature a substantialincrease can be obtained in the desirable stress patterns developed.

Another indication of the improved results of quenching according tothis invention is shown in the increased resistance of the wheels toexplosive failure during dragbrake testing. In drag-brake testing anotch is machined in the rim of the wheel to simulate a thermal-crack.The wheel is then subjected to drag-braking cycles until the wheel failsexplosively or until excessive tread wear precludes further testing.Each drag-braking test is conducted at a constant speed of 45 mph. withdiametrically opposed cast-iron brake shoes being applied for fiftyseconds out of each minute for a tolal of thirty-five minutes. The wheelis then cooled for a period of thirty minutes. Table II below comparestest results obtained on a Class B and Class C composition rim-toughenedrailroad wheels each of which had a two minute rim-quench and a fiveminute plate-hub portion quench having a fifteen seconds delay aftertempering with conventionally rim-toughened railroad wheels of Class Band Class C compositions slow-cooled after tempering. The number oftests needed to produce explosive failure for the conventional wheels isthe average of several tests on several wheels with the maximum numberof tests for the Class B composition being about forty-five and for theClass C composition about twenty-five.

TABLE H No. of Drag- Braking Tests Heat Treatment Composition to Producean Explosive Failure Rim-Toughened: Slow-Cooled after Tempering 1 B 40(Average) Do 1 C 15 (Average) Rim Toughened: Quenched after Tempering, 5Min. Plate-Hub Quench, 2 Min. Rim-Quench, 15 Sec. Delay 1 B 2 70+ 1Class B composition: 0 0.570.67%; Mn, 0.60-0.85% P 0.05% Max.; S, 0.05%Max.; Si, 0.15% lVIin. Class O composition: (,1, 0.67-0.7775; Mn,0.60-0.85%; P, 0.05% Max.; 8, 0.05% Max.; Si, 0.15% Min.

2 Did not fail after 70 tests; testing discontinued due to tread wear.

The results shown in Table II above clearly indicate the enhancedresistance to explosive failure of wheels treated according to thisinvention.

It has been determined that when quenching according to this inventionit is critical that the quench of the rim portion start at least as soonas the quench of the platehub portion. It is also critical that theplate-hub portion be quenched with sufficient rapidity to reach ambienttemperature before the rim portion reaches ambient temperature. Whenthese two critical conditions exist there will be inherently developeddesirable stress patterns in both the rim and the plate-hub portions.However, by varying the rates of flow of the quenching media, the

delay times, and the total time of quenching of the rim the actualstress pattern can be varied to give the most desirable combination ofhoop compressive stresses in the rim portion and compressive stresspatterns in the plate portion. It is even possible by selecting properflow rates to start quenching both the rim portion and the plate-hubportion together but of course the plate-hub portion must reach ambienttemperature before the rim portion does, as was indicated above. It willbe apparent that by varying the variables listed above difierent stresscombinations and patterns can be obtained depending upon the balancerequired between rim stresses and critical fillet stresses.

One of the outstanding advantages of this method of differentialquenching resides in the fact that the quenching is used solely toproduce desirable stress patterns. The variation of the quenching ratesand delays will not result in any material change in the micro-structureof the wheel developed at the selected tempering temperature. Thus,there need be no consideration given to any possibility of obtainingundesirable physical properties it the quenching rates and times arevaried. This is in contra-distinction to the prior art practices ofquenching from above the austenitizing temperature and attempting tocontrol the stress patterns by this quenching. Since rate of quenchingand delays in quenching from above the austenitizing temperature affectthe micro-structure developed upon quenching from any given temperatureabove the critical temperature this quenching in the prior art fromabove the austenitizing temperature must be designed to produce adesirable, or at least an acceptable, micro-structure; and, anyquenching method which does not produce an acceptable micro-structure isnot acceptable irrespective of how beneficial the resulting stresspattern may be. Thus, with prior art methods very real limits wereimposed upon the quenching methods with the result that they could notbe changed if such a change resulted in an undesirable micro-structure;and even if changes could be made the result was a less thansatisfactory compromise between the micro-structure and stress patterns.However, micro-structure need be given no consideration when workingwithin the limits defined in this invention.

It has further been determined that the desirable stress patterns ofthis invention are achieved irrespective of the previous hardeningtreatment. The desirable stress patterns are produced if the wheel hasbeen rim-toughened or oil-quenched from the austenitizing temperature.

While one embodiment of my invention has been shown and described itwill be apparent that other adaptations and modifications may be madewithout departing from the scope of the following claims.

I claim:

1. A method of heat-treating after hardening a steel railroad wheelhaving a rim portion and a plate-hub portion, which method comprises thesteps of heating the wheel to a temperature between 600 F. and the lowercritical temperature of the steel, differentially quenching both the rimportion and the plate-hub portion from said temperature, said quenchingbeing characterized by the quench of the rim portion commencing at leastas soon as the quench of the plate-hub portion, and the quench of theplate-hub portion being sufficiently rapid to bring the plate-hubportion to ambient temperature before the rim portion reaches ambienttemperature.

2. The method of claim 1 wherein the quenching of the rim portion isceased before the rim reaches ambient temperature and thereafter the rimis air-cooled to ambient temperature.

3. A method of heat-treating after hardening a steel railroad wheelhaving a rim portion and a plate-hub portion, which method comprises thesteps of heating the wheel to a temperature between 850 F. and 950 F.,water spray quenching the rim portion from said temperature at the rateof about 10 gallons per minute and continuing said quench of the rim forabout two minutes, commencing to water spray quench the plate-hubportion between five and fifteen seconds after the start of the quenchof the rim portion, and continuing the quenching of the plate-hubportion for about five minutes at the rate of about 10 gallons perminute.

4. The method of claim 1 wherein quenching comprises water sprayapplication.

References Cited in the file of this patent UNITED STATES PATENTSHildorf Dec. 17, 1929 Walcher Oct. 29, 1935 OTHER REFERENCES

1. A METHOD OF HEAT-TREATING AFTER HARDENING A STEEL RAILROAD WHEELHAVING A RIM PORTION AND A PLATE-HUB PORTION, WHICH METHOD COMPRISES THESTEPS OF HEATING THE WHEEL TO A TEMPERATURE BETWEEN 600* F. AND THELOWER CRITICAL TEMPERATURE OF THE STEEL, DIFFERENTIALLY QUENCHING BOTHTHE RIM PORTION AND THE PLATE-HUB PORTION FROM SAID TEMPERATURE, SAIDQUENCHING BEING CHARACTERIZED BY THE QUENCH OF THE RIM PORTIONCOMMENCING AT LEAST AS SOON AS THE QUENCH OF THE PLATE-HUB PORTION, ANDTHE QUENCE OF THE PLATE-HUB PORTION BEING SUFFICIENTLY RAPID TO BRINGTHE PLATE-HUB PORTION TO AMBIENT TEMPERATURE BEFORE THE RIM PORTIONREACHES AMBIENT TEMPERATURE.